Aluminum alloy for casting-forging, aluminum casting-forging product and processes for production thereof

ABSTRACT

There is disclosed a cast-forged product of an aluminum alloy consisting essentially of: 0.6 to 1.8 wt % of silicon; 0.6 to 1.8 wt % of magnesium; 0.8 wt % or less of copper; 0.2 to 1.0 wt % of manganese; 0.25 wt % or less of chromium; 0.0 to 0.15 wt % of titanium; and unavoidably contained impurities. When the product is used as various parts for automobiles formed of aluminum, such as suspension parts, frames, and parts for engines, the product is more superior in mechanical properties such as a tensile strength, proof stress, and elongation, and can be manufactured with a low cost.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a cast aluminum alloy for forging andan aluminum cast-forged product that is used for a vehicular part or thelike and is less costly, and a method of manufacturing an aluminumcast-forged product. More particularly, it relates to a cast aluminumalloy for forging that is used to manufacture vehicular suspension partswhich are required to be light in weight to improve fuel consumption ofan automobile and can use useless forging materials such as flashgenerated in a forging process, an aluminum cast-forged product that hashigh mechanical properties and contains particular amounts of silicon,magnesium, copper, and manganese, and a method of manufacturing analuminum cast-forged product.

2. Description of the Related Art

It is said that global warming, which is one of global environmentalproblems, is greatly affected by carbon dioxide produced by all humanactivities, and a reduction in carbon dioxide discharged from factoriesand electric power plants and a reduction in fuel consumption ofautomobiles are strongly required all over the world. At the thirdconference of the United Nations Framework Convention on Climate Change,so-called Global Warming Prevention Conference COP3, held in Kyoto in1997, Japan promised that the emission of gas causing greenhouse effectwhich mainly contains carbon dioxide will be decreased by 6% on averageof values in 2008 through 2012 as compared with 1990. Based on thispromise, regarding the fuel consumption rate of automobiles, the targetreference value of fuel consumption rate under vehicle weightclassification was determined with the target fiscal year being fiscal2010 for gasoline engine and fiscal year 2005 for diesel engine. Also,in the taxation system, measures were taken to treat low-pollution carsfavorably. Hereafter, auto manufacturers will strongly be pressed topromote technology development to improve fuel consumption and todevelop an automobile having improved fuel consumption, with furtherprogress of understanding towards environmental problems by automobilepurchasers and users. Such development will be needed to win competitionamong the auto manufactures.

The measures for improving the fuel consumption of the automobileinclude the use of new power sources such as a fuel cell, natural gas,and electricity, or the hybrid use thereof, the technical improvement ina motor system such as a lean fuel engine and a direct injection engine,and the decrease of running resistance due to improvement in loss of apower transmission system and improvement in vehicle body contours.Among these measures, a measure that is most effective and capable ofbeing taken together with any other technology is the reduction inweight of the automobile. If the automobile itself is made light in theweight, the load on the power source is lessened, and the amount ofpower to be consumed can be saved irrespective of power sources. As onemeasure for the reduction in weight of the automobile, the reduction inweight of the suspension parts of the automobile is an object having ahigher priority, because this reduction contributes to the improvementin drivability and riding quality of the automobile. In recent years,frame parts or some parts of the engine have also been regarded as theobjects of the reduction in weight, and attempts have been made to uselight metal materials therefor.

Incidentally, when the lightening in the weight of automobile isintended, the up in the cost remains as a theme to be solved. The weightlightening technology is broadly divided into a structure designtechnology and a material technology. In comparison with the drasticimprovement in vehicle body structure and construction elements, thechange of material used is an easily employable measure for lighteningthe weight. However, such a material for lightening the weight isgenerally expensive. Examples of the material for lightening the weightinclude resin materials such as FRP, thinned iron sheets using hightensile strength steel sheets, aluminum alloys, magnesium alloys,titanium alloys, ceramics, metallic compound materials, and the like.Among these materials, aluminum alloys have fewest drawbacks such ascorrosion resistance and the like, and are lowest in cost in thematerials for lightening the weight although being higher in cost thaniron, and can easily be applied as alternatives without requiring anylarge change in the basic design of the automobile.

The aluminum alloys, having a density about one-third that of iron, havealready been used for many easy-to-manufacture castings such as enginecylinder heads and engine cylinder blocks. These castings aremanufactured by high-speed injection molding, a so-called high pressuredie casting method, and thus can be manufactured at a relatively lowcost with high production efficiency. However, castings having largethickness and high strength cannot be produced. There is a problem thatthe application of cast parts to suspensions as a part light in weightsince a failure of such a part due to insufficient strength directlyleads to the safety problem.

Referring to the suspension parts which have advanced in studies of theweight reduction technology as examples, the present status of thetechnology will be described. Materials used for the suspension partssuch as a steering knuckle and a suspension arm are required to havehigh corrosion resistance, sufficient properties such as strength andelongation, and few defects, and an A6061 alloy forged product, an AC4CHalloy squeeze cast product (low-speed injection molded product), and thelike that meet the requirements have already been used. However, thesematerials still have an unsolved problem of high cost, so that theapplication thereof is extremely limited at present.

Reasons why so-called aluminum products formed only by forging, such asthe conventional A6061 alloy forged product are high in cost lie in thatthe number of manufacturing processes is large and that the raw materialfor forging itself is expensive and that wastes such as flash are causedduring the manufacturing process and that useless materials such as theflash cannot be recycled as the raw material for forging. Also, for asqueeze casting, because of a large number of processes and a lowinjection speed, the productivity is low and the cost reduction cannotbe attained.

Thus, in particular, in order to reduce the weight of the vehicularpart, an aluminum product having superior corrosion resistance,strength, and elongation, no defects, and a low cost has been demanded.To meet this demand, various improved aluminum alloys have heretoforebeen proposed as a material for manufacturing an aluminum product.

According to JP-A-5-59477, an aluminum alloy for forging in which thecoarseness of crystal grains is restrained by controlling a composition,whereby high mechanical properties are attained has been proposed. It isstated that a tensile strength of 40 kgf/mm² has been attained becauseof the improvement in strength of matrix, and the restraint in notcoarsening crystal grains by adjusting the composition so as to contain1.0 to 1.5 wt % of silicon, 0.8 to 1.5 wt % of magnesium, 0.4 to 0.9 wt% of copper, 0.2 to 0.6 wt % of manganese, 0.3 to 0.9 wt % of chromium,and the like.

Although the strength is increased, there arises a new problem that alow cost cannot be attained, and the corrosion resistance isdeteriorated because a larger amount of copper is contained than theconventional raw material for forging (A6061 alloy), and also thefluidity decreases so that the castability is poor because muchmagnesium is contained.

Also, an aluminum alloy material for forging having superior castabilityand high strength has been proposed in JP-A-7-258784. According to thisdocument, the formation of crack during casting, which has been formedin the case where the conventional A6061 alloy is used as the rawmaterial, does not occur, in the case of aluminum alloy forged productobtained by casting continuously a molten metal of an aluminum alloymaterial in which the composition is adjusted so as to contain 0.8 to2.0 wt % of silicon, 0.5 to 1.5 wt % of magnesium, 0.5 to 1.0 wt % ofcopper, 0.4 to 1.5 wt % of manganese, 0.1 to 0.3 wt % of chromium, andthe like with controlling a cooling rate in a solidification process,thereafter soaking the resultant, subsequently hot-forging aluminumalloy, thereafter subjecting to a solution heat treatment, and furtheran aging treatment, when the aluminum alloy forged product is cast intoa shape close to a final product.

In this proposal, although castability is improved, the low cost cannotbe attained yet as compared with the conventional raw material forforging (A6061 alloy), also the corrosion resistance is deterioratedbecause much copper is contained, and there remains anxiety when thismaterial is used for the suspension part. Also, there arises a problemthat the fluidity decreases because much magnesium is contained, and theabove-described rigorous control is needed in the casting process, andthe manufacturing cost rather increases.

Furthermore, according to JP-A-8-3675, an aluminum alloy for forginghaving superior mechanical properties and involving the low cost hasbeen proposed. It is stated that the formation of the hot crack does notoccur at the time of casting, and that the strength after the forgingcan be improved by forging, with an upsetting ratio of 10 to 50%, analuminum alloy whose components have been adjusted so as to contain 0.6to 3.0 wt % of silicon, 0.2 to 2.0 wt % of magnesium, 0.3 to 1.0 wt % ofcopper, 0.1 to 0.5 wt % of manganese, 0.1 to 0.5 wt % of chromium, andthe like, and also 1.5 wt % or more of Mg₂Si.

In this proposal, although a shape close to that of the final productcan be formed at the time of the casting, and the manufacturing cost canbe reduced because the forging can be performed by omitting an extrusionprocess, there arises a problem in that the strength decreases becausean excessive amount of manganese is contained. Manganese is an elementcapable of restraining the growth of aluminum crystal grains, keepingthe gain structure to be refined, and improving the strength. If thecontent thereof is high, however, intermetallic compounds are liable tobe formed, and the strength is rather decreased.

In JP-A-2002-302728, the present inventors also have proposed athick-wall aluminum processed product which is an aluminum cast-forgedproduct having high tensile strength, proof stress, and elongation andhaving improved mechanical properties as compared with the conventionalcast-forged product, and having superior corrosion resistance and highquality without any defects and involving the low cost, and a method ofmanufacturing the product. In this proposal, there is proposed, as acast aluminum alloy for forging which is the raw material for theforging, an aluminum alloy, characterized in that the alloy contains 0.2to 2.0 wt % of silicon, 0.35 to 1.2 wt % of magnesium, 0.1 to 0.4 wt %of copper, and 0.01 to 0.08 wt % of manganese.

When the above-described material is used, a desired effect can beattained. However, in respect of the mechanical strength, it cannot besaid that the material can sufficiently satisfy needs of the market,depending on use conditions. It is the present situation that therestill exists a demand for a material more superior in the mechanicalstrength.

There has been a demand for a low-cost aluminum product which is moresuperior in mechanical properties such as the tensile strength, proofstress, and elongation and which can be applied as various componentsfor automobiles such as the suspension components, frames, and engineparts and which involves the low cost, but it is the present situationthat an appropriate aluminum product has not been proposed yet.

SUMMARY OF THE INVENTION

The present invention has been developed in view of the above-describedconventional problems, and an object thereof is to solve the problemswith the conventional art and, more particularly, to provide an aluminumcast-forged product capable of satisfying needs of the market as analuminum thick-wall processed product and having high tensile strength,proof stress, and elongation; and a method of manufacturing an aluminumcast-forged product. That is, the object is to provide an aluminum alloymaterial which can be cast/processed like high-concentration productssuch as AC4CH containing 3 wt % or more of silicon and which can beprocessed in a final shape of a desired component like AC4CH withoutrequiring low-speed casting, an aluminum cast-forged product cast/forgedby the material, and a method of manufacturing an aluminum cast-forgedproduct. Another object of the present invention is to provide variouslightweight parts for vehicles brought about by the above-describedaluminum cast-forged product and the method of manufacturing theproduct, and accordingly to save fuel consumption of automobiles and toreduce the amount of emitted carbon dioxide and to contribute toenvironmental measures such as the prevention of global warming.

As a result of various studies on raw materials and manufacturing methodfor the thick-wall aluminum product to solve the above problems, thepresent inventors have found that an aluminum cast-forged product havinga sufficient strength capable of meeting needs of market can be obtainedby making predetermined amounts of silicon, magnesium, copper,manganese, and chromium, and optionally titanium contain therein, withimproving fluidity and castability. Thus, the present invention has beencompleted.

That is, according to the present invention, there is provided a castaluminum alloy for forging which is usable for a material for forging,consisting essentially of: 0.6 to 1.8 wt % of silicon; 0.6 to 1.8 wt %of magnesium; 0.8 wt % or less of copper; 0.2 to 1.0 wt % of manganese;0.25 wt % or less of chromium; 0.0 to 0.15 wt % of titanium; andunavoidable impurities. It is possible to use the present cast aluminumalloy for forging in manufacturing various parts for vehicles includingsuspension parts having mechanical properties capable of satisfyingneeds of the market such as a tensile strength of 320 MPa or more, aproof stress of 280 MPa or more, and an elongation of 10% or more.

To use the material in manufacturing a desired final product, assumingthat a shape of the final product is 100%, the aluminum alloy is used tocast a preformed product having a forging ratio of 18 to 60%, andsubsequently the preformed product may be cast and formed in the shapeof the final product. Accordingly, it is possible to manufacture theparts for vehicles with a lower cost and higher productivity as comparedwith low-speed casting using AC4CH.

Moreover, according to the present invention, there is also provided analuminum cast-forged product which is manufactured by forging apreformed product cast from the above-described aluminum alloyconsisting essentially of 0.6 to 1.8 wt % of silicon, 0.6 to 1.8 wt % ofmagnesium, 0.8 wt % or less of copper, 0.2 to 1.0 wt % of manganese,0.25 wt % or less of chromium, 0.0 to 0.15 wt % of titanium, andunavoidable impurities, the aluminum cast-forged product consistingessentially of: 0.6 to 1.8 wt % of silicon; 0.6 to 1.8 wt % ofmagnesium; 0.8 wt % or less of copper; 0.2 to 1.0 wt % of manganese;0.25 wt % or less of chromium; 0.0 to 0.15 wt % of titanium; andunavoidable impurities. Therefore, the manufactured aluminum cast-forgedproduct has a sufficient mechanical properties such that the product isusable as the suspension parts for the vehicles, the frames for thevehicles, and the parts for the engines.

Furthermore, according to the present invention, there is provided amethod of manufacturing an aluminum cast-forged product consistingessentially of 0.6 to 1.8 wt % of silicon, 0.6 to 1.8 wt % of magnesium,0.8 wt % or less of copper, 0.2 to 1.0 wt % of manganese, 0.25 wt % orless of chromium, 0.0 to 0.15 wt % of titanium, and unavoidableimpurities, the method including: a melting step of melting a materialfor forging which is an aluminum alloy consisting essentially of 0.6 to1.8 wt % of silicon, 0.6 to 1.8 wt % of magnesium, 0.8 wt % or less ofcopper, 0.2 to 1.0 wt % of manganese, 0.25 wt % or less of chromium, 0.0to 0.15 wt % of titanium, and unavoidable impurities at about 680 to780° C. to obtain a molten metal; a casting step of casting the obtainedmolten metal at a mold temperature of about 60 to 150° C. to obtain araw material for forging; a rough forging step of heating the rawmaterial for forging to a surface temperature at about 380° C. to amelting point or less and forging the raw material to obtain a roughlyforged material; a finish forging step of heating the roughly forgedmaterial to a surface temperature at about 380° C. to the melting pointor less and forging the roughly forged material to obtain a finishforged material; and a clipping flash step of removing flash from thefinish forged material to obtain a final product.

For the material for forging, while the respective components areadjusted so as to obtain the composition of the aluminum alloy accordingto the present invention, the flash generated at the time of the forgingmay be reused as the raw material. It is to be noted that assuming thatthe shape of the final product is 100%, a shape forging ratio of the rawmaterial for forging, that is, the preformed material is preferably 18to 60%. Therefore, it is possible to preferably manufacture thesuspension parts for the vehicles, the frames for the vehicles, and theparts for engine by the method of manufacturing an aluminum cast-forgedproduct according to the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view showing one example of an aluminum cast-forgedproduct in accordance with the present invention;

FIGS. 2(a), (b), and (c) are views showing one example of a method ofmanufacturing an aluminum cast-forged product of the present invention,FIG. 2(a) is a schematic explanatory view showing a difference in shapeof a preformed material for each forging ratio at the time of casting,FIG. 2(b) is an enlarged side view showing one example of a cast body inwhich an internal defect is generated at the time of the casting, andFIG. 2(c) is an enlarged side view showing one example of a cast body inwhich no internal defect is generated at the time of the casting; and

FIGS. 3(a) and (b) are sectional views of the preformed material showingthe forging ratio.

The numerical references used in the drawings denote respectively apart, an apparatus, a portion or the like as specified below:

21, 22 . . . columnar test piece, 40 . . . steering knuckle, 41, 42, 43,44 . . . position from which test piece was taken, and 50 . . . internaldefect.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Hereunder, embodiments of a cast aluminum alloy for forging, an aluminumcast-forged product, and a method of manufacturing an aluminumcast-forged product in accordance with the present invention will bedescribed in detail. The present invention is not construed by beinglimited to these embodiments, and various changes, modifications, andimprovements can be made based on the knowledge of those skilled in theart as far as such changes, modifications, or improvements are withinthe scope of the invention.

In the present invention, an aluminum alloy consisting essentially of0.6 to 1.8 wt % of silicon, 0.6 to 1.8 wt % of magnesium, 0.8 wt % orless of copper, 0.2 to 1.0 wt % of manganese, 0.25 wt % or less ofchromium, 0.0 to 0.15 wt % of titanium, and unavoidable impurities isused to first cast a preformed material, and next the preformed materialis forged to manufacture a cast-forged product having a desired shape.When the aluminum alloy constituted of the above-described compositionis used, it is possible to prepare an aluminum cast-forged product ofthe present invention, having mechanical properties meeting marketingneeds. It is possible to preferably use the product in parts forvehicles in rugged environments, especially suspension parts forautomobiles, frames for vehicles, and parts for engines.

The cast aluminum alloy for forging and the aluminum cast-forged productof the present invention will hereinafter concretely be described.

The cast aluminum alloy for forging of the present invention is analuminum alloy consisting essentially of 0.6 to 1.8 wt % of silicon, 0.6to 1.8 wt % of magnesium, 0.8 wt % or less of copper, 0.2 to 1.0 wt % ofmanganese, 0.25 wt % or less of chromium, 0.0 to 0.15 wt % of titanium,and unavoidable impurities.

Silicon serves to enhance fluidity and to improve a casting shrinkagewhen contained in the aluminum alloy. Also, this element precipitatesMg₂Si when coexisting with magnesium, and contributes to the improvementin mechanical properties such as elongation, tensile strength, and proofstress. When the content of silicon is less than 0.6 wt %, a sufficientmechanical property is not secured. On the other hand, when the contentof silicon exceeds 1.8 wt %, the elongation decreases, it is notpossible to manufacture the product in accordance with marketing needs,and therefore this is not preferable. It is to be noted that the contentof silicon is preferably 0.8 to 1.3 wt %, further preferably 0.8 to 1.1wt %.

Magnesium precipitates Mg₂Si in a matrix, when coexisting with silicon,and improves the mechanical properties such as the elongation, tensilestrength, and proof stress when contained in the aluminum alloy. Sincethe present invention provides an aluminum cast-forged productsubstituted for the conventional forged product although being low incost, the strength higher than that of the conventional product isindispensable, and magnesium needs to be contained. However, even ifmuch magnesium is contained, there is no increase in strength. With toomuch content, since magnesium is an element liable to be oxidized,oxidation of molten metal is accelerated, the fluidity decreases, andthe casting defect is liable to be generated. Also, the corrosionresistance is deteriorated, so that the product cannot withstand harshservice environments. Therefore, a rather low content is preferable.

It is preferable that 0.6 to 1.8 wt % of magnesium be contained in thecast aluminum alloy for forging. If the content of magnesium is lessthan 0.6 wt %, the amount of precipitation of Mg₂Si is undesirablyinsufficient, and the strength is insufficient. If the content is morethan 1.8 wt %, in addition to the insufficient strength, quenchingsensitivity decreases, and thus the forging defect is liable to begenerated. As a result, the quality of the forged material decreases,and the mechanical property also unfavorably decreases. The content ispreferably 0.6 to 1.2 wt %, further preferably 0.7 to 1.1 wt %.

Copper is an element that can improve the strength, when contained inthe aluminum alloy. For a copper-containing forged material, an Al—Cu orAl—Cu—Mg based precipitate yielded by a so-called aging treatment, inwhich the forged material is left to stand at ordinary temperature aftercooled and crystals are precipitated for a long period of time, can beobtained. Accordingly, a strength improving function by Mg₂Siprecipitated as described above is promoted to enhance the strength. Inthe present invention, since the strength higher than that of theconventional forged material is indispensable, it is preferable thatcopper be contained. However, in consideration of an application toproducts in which corrosion resistance is regarded as most important,such as an automobile suspension part, if too much copper, liable to beoxidized, is contained, the forged material is easily corroded, and itis therefore preferable that the content of copper be controlled to beas low as possible.

It is preferable that 0.8 wt % or less of copper be contained in thecast aluminum alloy for forging. If the content of copper is more than0.8 wt %, the corrosion resistance is deteriorated, the alloy is liableto rust, and the strength cannot unfavorably be maintained over a longperiod. The content is preferably 0.005 wt % or more, less than 0.3 wt%, further preferably more than 0.1 wt %, and less than 0.2 wt %.

Manganese is an element that restrains the recrystallization of thealuminum alloy and the growth of crystal grains, when contained in thealuminum alloy. As a result, the grain structure in the aluminum alloyis kept to be refined, and the strength is maintained. In the presentinvention, since it is necessary to maintain the mechanical propertiessuch as the elongation, tensile strength, and proof stress over the longperiod, a minute amount of manganese needs to be contained. However, iftoo much manganese is contained, workability decreases at the time ofthe forging, also intermetallic compounds are formed, and a decrease inthe mechanical properties, especially the elongation, is found.

It is preferable that 0.2 to 1.0 wt % of manganese be contained in thecast aluminum alloy for forging. If the content of manganese is lessthan 0.2 wt %, a desired strength cannot sometimes be obtained. If thecontent is more than 1.0 wt %, formability undesirably decreases, anddefects are liable to be generated. The content is more preferably morethan 0.5 wt %, and 0.7 wt % or less.

Chromium forms dispersed particles, and it has an effect of inhibiting agrain boundary from moving after recrystallization, when contained inthe aluminum alloy. Therefore, refined crystal grains and sub-crystalgrains can be obtained. It is preferable that 0.25 wt % or less ofchromium be contained in the cast aluminum alloy for forging. Even whenthe content of chromium exceeds 0.25 wt %, the desired effect cannotsometimes unfavorably be attained. The content is more preferably 0.04to 0.25 wt %.

Titanium refines the crystal grains of a casting, and enhancesworkability at the time of the forging, when contained in the aluminumalloy. It is preferable that 0.0 to 0.15 wt % of titanium be containedin the cast aluminum alloy for forging. It is to be noted that even whentitanium is not contained, a considerably large trouble is not caused.

The metals contained in minute amounts in the cast aluminum alloy forforging and the aluminum cast-forged product in accordance with thepresent invention are as described above, and the balance is unavoidablycontained impurities and aluminum. It is preferable that the unavoidablycontained impurities be contained by an amount as small as possible. Thecontent is less than 0.1 wt %, preferably 0.05 wt % or less.

It is to be noted that in a casting/forging method according to thepresent invention, flash generated in the forging process and accountingfor about 30% of the generally used raw material can be recovered andreused as the raw material of the aluminum alloy according to thepresent invention. Therefore, in the present invention, the cost of rawmaterials can be reduced.

Next, the method of manufacturing an aluminum cast-forged product of thepresent invention will be described.

As described above, it is preferable that the flash generated at thetime of the forging be used as the raw material. This raw material isadjusted so as to allow the resulting composition to form the intendedaluminum alloy consisting essentially of 0.6 to 1.8 wt % of silicon, 0.6to 1.8 wt % of magnesium, 0.8 wt % or less of copper, 0.2 to 1.0 wt % ofmanganese, 0.25 wt % or less of chromium, 0.0 to 0.15 wt % of titanium,and unavoidably contained impurities by measures such as preparing ofmetals that are insufficient using pure metals, and mixing with anotheraluminum alloy. At this time, it is preferable that the unavoidableimpurities be not contained in the aluminum alloy by 0.1 wt % or more intotal.

These raw materials are charged in a melting furnace and is heated to atemperature at about 680 to 780° C. to be melted, and next is chargedinto a holding furnace where a degassing treatment and deoxidizingtreatment are done to obtain a molten metal. In this case, thetemperature of a mold is preferably adjusted at about 60 to 150° C.Also, the mold preferably has a shape such that a forging ratio is about18 to 60%, assuming that the shape of the final forged product is 100%,because the strength is enhanced by the subsequent forging and theforging process can further be simplified. That is, when the forgingratio is set to about 18 to 60%, the strength improving effect due tothe forging and the cost reduction due to a simplified forging processare balanced.

Herein, the forging ratio means a value representing the degree offorming. For example, as shown in FIG. 3(a), when a material A with aninitial thickness D1 is formed by a load F and the thickness is changedto D2 after forming as shown in FIG. 3(b), a forging ratio R isrepresented by the following equation.R[%]=(D 1−D 2)/D 1×100 (D 1>D 2)However, when the thickness D2 after the forming is larger than theinitial thickness, the forging ratio is represented by the followingequation.R[%]=(D 2−D 1)/D 1×100 (D 2>D 1)

That is, in the present invention, the fact that the so-called preformedmaterial having a shape such that the forging ratio is about 18 to 60%assuming that the shape of the final forged product is 100% is obtainedby the casting means that the preformed material having a shape suchthat the forging ratio determined using the thickness of each portion ofthe raw material for forging and the thickness of each correspondingportion in the final product obtained by forging the raw material forforging is about 18 to 60% in each portion is obtained by the casting.

Next, the cast material obtained/molded using a casting apparatus, thatis, the raw material for forging is heated to a surface temperature atabout 380° C. to a melting point or less and is stamped by a forgingpress to obtain a roughly forged material. The roughly forged materialis cooled, thereafter heated again to the surface temperature at about380° C. to the melting point or less, and is finished/stamped by theforging press to obtain a finish forged material. The finish forgedmaterial is subjected to clipping flash and heat treatment such as T6treatment to obtain a forged product. For example, to manufacture asteering knuckle which is one of the suspension parts for theautomobiles, the load of the forging press is about 2600 to 2800 tonsfor rough forging and about 3200 to 3800 tons for finish forging. Bythis manufacturing process, the aluminum cast-forged product inaccordance with the present invention can be obtained.

In the present invention, the flash generated by the forge-pressing andclipping flash in the manufacturing process of the present invention iscollected by a flash removing machine, and can be reused as the rawmaterial for the aluminum cast-forged product of the present invention.Therefore, all of the raw materials for forging are recycled, and arenot disposed of as wastes or are not used as an inexpensive raw materialfor forging.

In the method of manufacturing an aluminum cast-forged product of thepresent invention, after the raw material is melted to obtain the moltenmetal, the mold for the casting is brought closer to the shape of theproduct as compared with the conventional raw material for forging sothat the forging ratio is about 18 to 60% assuming that the shape of thefinal forged product is 100% while achieving the strength improvingeffect by the forging, by which the pressing is facilitated. Therefore,unlike the conventional forging process, steps of extruding, cutting,heating, rough forging, finish forging, and clipping flash are notobserved, thus the manufacturing process can be simplified, and themanufacturing cost can be reduced.

EXAMPLES

Examples of the present invention will hereinafter be described, but thepresent invention is not limited to these examples.

Examples 1 and 2

FIG. 1 is a diagram showing one example of the aluminum cast-forgedproduct of the present invention. A steering knuckle 40 which is thepart for the automobile is shown. A small amount of copper was added toscraps of A6082 alloy to prepare a raw material forming the aluminumalloy consisting essentially of 0.6 to 1.8 wt % of silicon, 0.6 to 1.8wt % of magnesium, 0.8 wt % or less of copper, 0.2 to 1.0 wt % ofmanganese, 0.25 wt % or less of chromium, 0.0 to 0.15 wt % of titanium,and unavoidably contained impurities. Using this raw material, thesteering knuckle 40 having a shape shown in FIG. 1 was manufactured inaccordance with the following steps.

After the raw material was melted at a molten metal temperature of 728°C. to obtain the molten metal, the raw material for forging having ashape with a forging ratio of 30% assuming that the shape of the finalsteering knuckle 40 was 100% was cast at a mold temperature of 100° C.Next, die forging was performed using a forging press at a rough forgingtemperature of 395° C. (surface temperature) by applying a rough forgingload of 2770 tons to obtain a roughly forged material. Then, the roughlyforged material was subjected to the die forging again using the forgingpress at a finish forging temperature of 460° C. (surface temperature)by applying a finish forging load of 3260 tons. Finally, the finishforged material was trimmed. After a solution heat treatment which was aT4 treatment by heating the finish forged material at 530° C. for threehours, the finish forged material was cooled. Then, an aging treatmentwas done as T6 treatment by heating the finish forged material at 180°C. for six hours. Thus, the steering knuckle 40 was obtained as aproduct. In Example 2, operations similar to those of Example 1 wererepeated to obtain the steering knuckle 40, except that the molten metaltemperature was set to 720° C. and the mold temperature was set to 125°C. Temperature conditions and load conditions are given in Tables 1 and2, respectively. TABLE 1 Molten metal temperature Mold temperature (°C.) (° C.) Example 1 728 100 Example 2 720 125

TABLE 2 Rough forging Finish forging Surface Surface temperaturetemperature Load (ton) (° C.) Load (ton) (° C.) Example 1 2770 395 3260460 Example 2 2730 400 3780 445

The test pieces were cut out from thus obtained steering knuckle 40, andtensile strength, proof stress, and elongation were measured asmechanical properties. The results are shown in Table 3. TABLE 3 TensileProof strength stress Elongation (MPa) (MPa) [%] Example 1 358 323 14.7Example 2 378.6 335.8 14.3 Required value 320 280 10 from market

The results of Examples 1 and 2 reveal that the mechanical properties ofthe aluminum cast-forged product in accordance with the presentinvention were capable of clearing the standards as needs of market inall of the tensile strength, proof stress, and elongation.

As described above, in accordance with the present invention, there isprovided an aluminum cast-forged product further satisfying needs ofmarket in mechanical properties such as a tensile strength, proofstress, and elongation with a simpler manufacturing process with goodproductivity and at low cost. Moreover, by this aluminum cast-forgedproduct, various lightweight and inexpensive vehicular parts such assuspension parts for vehicles, frames for the vehicles, and parts forengines are provided. Through lightening of weights of the vehicles,fuel consumption of automobiles is saved. As a result, emitted carbondioxide is reduced, and an effect of contribution to prevention ofglobal warming is also attained.

1. (canceled)
 2. A use of an aluminum alloy in manufacturing an aluminumcast-forged product, wherein a preformed material is cast from thealuminum alloy consisting essentially of 0.6 to 1.8 wt % of silicon, 0.6to 1.8 wt % of magnesium, 0.8 wt % or less of copper, 0.2 to 1.0 wt % ofmanganese, 0.25 wt % or less of chromium, 0.0 to 0.15 wt % of titanium,and unavoidably contained impurities, and the preformed material isforged to manufacture an aluminum cast-forged product.
 3. The use inmanufacturing an aluminum cast-forged product according to claim 2,wherein the preformed material has a shape indicating a forging ratio Rof 18 to 60%, wherein the forging ratio is given by either one offollowing equations:R[%]=(D 1−D 2)/D 1×100 (D 1>D 2), orR[%]=(D 2−D 1)/D 1×100 (D 2>D 1).
 4. The use in manufacturing analuminum cast-forged product according to claim 2, wherein the aluminumcast-forged product is a suspension part for a vehicle, a frame for thevehicle, or a part for an engine.
 5. The use in manufacturing analuminum cast-forged product according to claim 3, wherein an aluminumcast-forged product is a suspension part for a vehicle, a frame for thevehicle, or a part for an engine.
 6. A method of manufacturing analuminum cast-forged product consisting essentially of 0.6 to 1.8 wt %of silicon, 0.6 to 1.8 wt % of magnesium, 0.8 wt % or less of copper,0.2 to 1.0 wt % of manganese, 0.25 wt % or less of chromium, 0.0 to 0.15wt % of titanium, and unavoidable impurities, the method including: amelting step of melting an aluminum alloy consisting essentially of 0.6to 1.8 wt % of silicon, 0.6 to 1.8 wt % of magnesium, 0.8 wt % or lessof copper, 0.2 to 1.0 wt % of manganese, 0.25 wt % or less of chromium,0.0 to 0.15 wt % of titanium, and the unavoidable impurities at about680 to 780° C. to obtain a molten metal; a casting step of casting theobtained molten metal at a mold temperature of about 60 to 150° C. toobtain a preformed material which is a raw material for forging; a roughforging step of heating the raw material for forging to a surfacetemperature at about 380° C. to a melting point or less and forging theraw material to obtain a roughly forged material; a finish forging stepof heating the roughly forged material to a surface temperature at about380° C. to the melting point or less and forging the roughly forgedmaterial to obtain a finish forged material; and a clipping flash stepof removing flash from the finish forged material to obtain a finalproduct.
 7. The method according to claim 6, wherein the aluminum alloyincludes the flash generated at the time of the forging as a portion ofa raw material.
 8. The method according to claim 7, wherein a forgingratio R of the shape of a preformed material is in a range of 18 to 60,wherein the forging ratio R is given by either one of followingequations:R[%]=(D 1−D 2)/D 1×100 (D 1>D 2), orR[%]=(D 2−D 1)/D 1×100 (D 2>D 1). 9.-14. (canceled)